Eleven people have been killed in clashes between supporters and opponents of the ousted president since Monday. Most were killed in pre-dawn street battles near a pro-Morsi protest camp as the country remained mired in turmoil three weeks after the military overthrew the Islamist leader.

A pro-Morsi group claimed Wednesday another two people were killed in a march in Cairo by assailants who fired on them from rooftops. Police and health officials could not immediately be reach for confirmation.

The bloodshed is widening the divisions between Morsi's supporters and the military-backed administration and diminishing the chances of reconciliation.

The police force, widely hated for its brutality and widespread abuses over the years, has been the target of fierce attacks in Egypt's volatile northern Sinai Peninsula. More than a dozen security officials have been killed there since Morsi's ouster earlier this month. A minor explosion recently struck a police post between Cairo and northern Sinai.

Wednesday's bomb explosion appeared to target police in the provincial capital city of Mansoura in the delta province of Dakahliya. It raised the specter that indiscriminate attacks targeting security forces could expand to larger cities beyond the usual targets in northern Sinai.

Security officials said 19 people were wounded, 13 policemen and six civilians, when the bomb outside the security directorate in Mansoura exploded after midnight. The city was bustling with people as is common during the Islamic month of Ramadan, when Muslims fast during the day and stay up late to eat and pray.

Police officials exchanged gunfire with unidentified people inside a nearby abandoned building afterward. No further details were immediately available.

Presidential spokesman Ahmad al-Muslimani released a statement calling the incident an act of terrorism.

"The Mansoura terrorist incident will not waver Egypt's resolve," he said. "Egypt has triumphed in the war against terrorism before and will win again today."

Morsi's Muslim Brotherhood party condemned the attack in a statement posted on the group's website and vowed to hold peaceful protests, saying they would not be dragged into violence.

Morsi's continued detention has fueled anger among his supporters. Egypt's first freely elected leader has been held incommunicado and without charge.

Yomna Ahmed, from the Anti-Coup Coalition that works closely with the Brotherhood, said two pro-Morsi protesters were shot dead by snipers on rooftops early Wednesday during a march in a residential neighborhood of Cairo.

Running street battles that erupted the night before, just before dawn on Tuesday, were among the most intense since the crisis began on July 3.

Clashes broke out after Morsi supporters began marching from their sit-in outside the main campus of Cairo University to a nearby mosque. The protesters blocked roads, causing massive traffic jams and angering residents.

Security officials said the fighting turned deadly after masked gunmen appeared and started shooting at the Morsi supporters with live ammunition and birdshot. The officials had no word on the identity of the gunmen. All of the officials spoke on condition of anonymity because they were not authorized to release the information.

The Muslim Brotherhood, however, blamed the killings on "thugs" sponsored by the Interior Ministry, a charge the Islamist group from which Morsi hails often uses to dismiss the notion that it was at odds with other segments of the population.

Khaled el-Khateeb, who heads the Health Ministry's emergency and intensive care department, said six people were killed near the pro-Morsi sit-in. The security officials put the casualty toll at seven killed and 11 injured.

Morsi's ouster followed massive street protests by millions of Egyptians demanding that the Islamist president step down. His supporters are calling for his reinstatement and insist they will not join the military-backed political process until then.

The latest violence underlines the depth of the polarization in Egypt. The deposed president's family denounced the military in a Monday news conference, accusing it of "kidnapping" him, and European diplomats urged that he be released.

In a separate development, two rights groups -- Human Rights Watch and Amnesty International -- urged Egyptian authorities to investigate a spate of attacks against Christians following Morsi's ouster and bring their perpetrators to account.

At least six Christians have been killed and scores injured in at least six provinces since July 3. The worst was in a village near the ancient city of Luxor, where four Christians were killed and three injured at the hands of a mob of Islamists. Other attacks included the shooting death of a priest in the Sinai town of el-Arish and the destruction and looting of Christian homes and stores in Minya province south of Cairo. A church was also targeted in Minya.

"A thorough, impartial and independent investigation must be conducted into the events in Luxor and the grossly inadequate response of the security forces to the attack," according to Amnesty.

"Authorities should hold accountable the people responsible for the sectarian killings and attacks on houses of worship and property, and investigate whether security forces took inadequate measures to prevent or stop the attacks," Human Rights Watch said.